The present invention, in some embodiments thereof, relates to material science, and more particularly, but not exclusively, to superabsorbent polymeric hydrogels and uses thereof.
Superabsorbent polymers are polymers that can absorb and retain extremely large amounts of a liquid relative to their own mass. When crosslinked and soaked with aqueous media (such as water), superabsorbent polymers are referred to as hydrogels that may absorb 800 times their dry weight and swell up to 100 times of their dry volume, to constitute up to 99.9% liquid. The total absorbency and swelling capacity are controlled by the type and degree of crosslinking of the polymer.
Superabsorbent crosslinked polymers are used commercially since 1970's in personal disposable hygiene products, diapers, sanitary napkins, blocking water penetration, horticultural water retention, spill and waste control, and even for making artificial snow.
The most common commercially-used superabsorbent polymers typically comprise polymerized alkaline acrylic acid that forms sodium polyacrylate. Other materials used for superabsorbent polymer include polyacrylamide copolymer, ethylene maleic anhydride copolymer, cross-linked carboxymethylcellulose, polyvinyl alcohol copolymers, cross-linked polyethylene oxide, and starch-grafted copolymer of polyacrylonitrile.
Presently known superabsorbent hydrogels are usually mechanically fragile in both the dry and swollen state, and most of these release the absorbed water when put under stress/pressure. To mitigate these problems, a number of relatively tougher hydrogels have been developed, some based on nanocomposite hydrogels that incorporate clay, and others based on double-network gels that combine a densely crosslinked ionic hydrogel with a lightly crosslinked non-ionic hydrogel. However, the increase in toughness often comes at the expense of water-uptake capacity.
Cipriano, B. H. et al., [Macromolecules, 2014, 47(13), pp 4445-4452] reported a superabsorbent hydrogels having superior mechanical properties based on self-crosslinking N,N-dimethylacrylamide (DMAA) and sodium acrylate.
High internal phase emulsions (HIPEs) are typically formed from two immiscible liquids, typically being water as a major dispersed or internal phase, and a highly hydrophobic liquid as a minor continuous or external phase. PolyHIPEs are highly porous polymeric structures formed by polymerization of monomers within the external phase of HIPEs. Thus, polyHIPEs are high internal phase emulsion-templated polymeric structures having highly interconnected porous microstructures. The microstructure of a polyHIPE consists primarily of large spheroidal voids, which are the structural remnants of the internal phase droplets, connected by “windows” where adjacent droplets were in close proximity. The size and shape of the voids are conferred by the droplets, and may be tens to hundreds of micrometers in diameter, whereas the interconnecting windows are in the hundreds to thousands of nanometers in diameter. For a review, see Feuerabendt, F. et al. [IJERR, 2014, 2(1), pp. 23-31].
Absorbent foams made from polyHIPEs are disclosed, for example, in U.S. Pat. Nos. 4,522,953, 5,550,167, 5,571,849, 5,633,291, 5,692,939, 5,728,743, 5,753,359, 5,770,634, 6,013,589, 6,048,908, 6,083,211, 6,147,131, 6,207,724, 6,362,243, 6,444,716, 6,525,106, 7,820,729, 8,921,435, 9,062,245 and 9,180,094.
Foams made from hydrophobic polyHIPEs are polymeric structures templated by water-in-oil (w/o) HIPEs. The water absorption expected in such hydrophobic polyHIPEs, typically characterized by porosities of 80 to 90%, is about 8 to 9 g/g (gram water to gram polymer).
Hydrophilic polyHIPE foams are polymeric structures templated by oil-in-water (o/w) HIPEs. Oil-in-water HIPEs comprising only ionic polymers (derived from ionic monomers) are difficult to stabilize through the polymerization process, and thus a compromise between pre-polymerization stability and pre-polymerization monomer composition restricts the hydrophilic polyHIPE available to date.
Krajnc and co-workers reported an oil-in-water HIPE consisting of acrylic acid, water, and a crosslinker (N,N′-methylene bisacrylamide) as the water phase, and toluene as the oil phase, which was successfully stabilized to sustain thermal initiation of radical polymerization to afford a porous open-cell monolithic material [Krajnc, P. et al., Macromol. Rapid Commun., 2005, 26, pp. 1289-1293]. This work was silent with respect to water absorption or mechanical properties of the resulting reticulated and highly porous foams.
Hydrophilic polyHIPE hydrogels based on a pre-polymerization mixture of non-ionic monomers, such as hydroxyethyl methacrylate (HEMA), and ionic monomers, such as methacrylic acid (MAA), were recently reported as having water absorption capacities of up to 18 g/g [Ovadia, M. and Silverstein, M. S., Polymer International, 2016, 65(3), pp. 280-289]. This unusually high water absorption was attributed to absorption that fills the original voids, swells the hydrogel walls, and fills the volume generated by hydrogel-swelling-driven void expansion.